Update example port - README.md (#19973)

The port number in the example is not the default and could be more clear.

Author: Azahorscak

keda/README.md

@@ -81,7 +81,7 @@ metadata:
           "init_config": {},
           "instances": [
             {
-              "openmetrics_endpoint": "http://%%host%%:8000/metrics"
+              "openmetrics_endpoint": "http://%%host%%:8080/metrics"
             }
           ]
         }